This invention relates to a gap-filling, adhesive composition for use with polymeric tie plates which are disposed in the dapped area of a wood, concrete or polyethylene railroad cross tie.
In the art of fastening railroad rails to cross ties, a metal tie plate is generally used between the rail and the cross tie. In U.S. Pat. No. 2,886,248 (Laudig) it was taught that a tie pad composition could be used between the metal tie plate and the wooden cross tie. The autoadhesive, autosealing tie pad composition contained: 45-65 wt. % asphalt, 5-25 wt. % heavy naphthalene base residuum, 3-12 wt. % pine tar and 10-35 wt. % of at least one rubbery elastomer. The asphalt has a penetration between 30 and 50 and a softening point between 100.degree. and 170.degree. F. (37.78.degree. C.-76.67.degree. C.). The naphthalene base has a viscosity of about 40-55 Saybolt Fural seconds at 210.degree. F. (98.89.degree. C.). The pine tar is a high grade retort tar made by the destructive distillation of wood. The rubbery elastomer can be a natural rubber, used rubber, or a synthetic rubber like a copolymer of butadiene and styrene (GR-S). The tie pad also may contain a fibrous membrane or filler but it may consist simply of the adhesive and sealant composition.
In U.S. Pat. No. 3,358,925 of which I was a co-inventor, there was disclosed a non-metallic tie plate which could be adhesively bonded to a wooden cross tie. The non-metallic plate consists of a plastic material like polyurethane or epoxy. The adhesive used to bond these non-metallic tie plates to the wood cross tie are resorcinol or epoxy adhesives. Also in my prior U.S. Pat. No. 3,544,006, I disclosed a non-metallic tie plate comprised of high density polyethylene. The high density polyethylene tie plate is disposed in a dapped area of a railroad cross tie and is usable with railroad spikes.
Polymers such as polyolefins have inherent shrink properties which cause the formation of slight sinks or pits in the base of the polyolefin tie plates during cooling after injection molding. These sinks make it difficult to have a good contact with other flat substrates like wood, concrete or polyethylene. When the substrate is wood, irregularities on the surface of the wood like checks, small splits, knots, worm holes, etc. further complicate optimum contact. Even if the polyolefin tie plates are molded in such a manner to produce a flat bottomed base with no sinks, damage may occur to the plates due to their movement while in service.
It has been found when polyolefin tie plates were attached directly to a wood or concrete cross tie that moisture, dust, and abrasive particles accumulate between the tie plate and the cross tie in the dapped area. Moisture present from rain, melting snow or condensation from the atmosphere, when the tie plate is cooler than the surrounding air, softens the topmost wood fibers of the cross tie. This softening makes it easier for the movement of the tie plate to abrade the wooded cross tie in the restrictive dapped area which acts as a catch-all. The dust and abrasive particles, like sand from the roadbed, lodge between the tie plate and the cross tie in the dapped area that acts as a receptacle and add to the deterioration and loss of service of the wooden cross tie. Also, with plates having flat bases, the movement of the tie plate while in service should be restricted as much as possible to prolong the life of the plate.
Some type of adhesive material with good adhesive strength is needed to fill the gap between, and also restrict the movement of, the polymeric tie plate and the wood or concrete or polyethylene cross tie. Adhesives containing 50-55% solids adhere fairly well to the polyolefin tie plate but these adhesives do not give adequate contact between the polyolefin tie plates and wood or concrete cross ties. This is caused by the loss of volume of the adhesive on drying. Even thick fluid solvent-containing adhesives may not give optimum contact between polyolefin tie plates and wood or concrete cross ties. Also, polyolefin tie plates are not naturally conducive to the use of several types of adhesives. Epoxy adhesive systems do not adhere satisfactorily to the waxy surface of polyolefin. Also, in the 100 percent polyurethane two component adhesive systems, component A, an isocyanate, reacts rapidly with wood moisture and moisture in concrete to form poor adhesive bonds. Also, epoxy and polyurethane adhesives, respectively, are two component systems which are troublesome to apply in the field, especially to cross ties in railroad tracks; and require expensive metering, mixing and dispensing equipment.
It is an object of this invention to provide a gap-filling, adhesive composition which gives optimum contact between the polymeric tie plate disposed in the dapped area of a wood, concrete or polyethylene cross tie and which is not dependent on chemical reactions but yet offers flexibility and good adhesive strength for long periods of time sufficient to hold the polymeric tie plate to the wood, concrete or polyethylene cross tie, especially in the dapped load bearing area.